In a cellular wireless communication system, an area is divided into cells, which are further divided into cell sectors (“sectors”). Each cell and cell sector is defined by a radiation pattern from a respective base transceiver station (BTS), which includes a radio-frequency antenna tower. Each BTS is then typically connected with a base station controller (BSC), which, together with the BTS, defines a “base station.” Further, the BSC may then be connected with a switch (e.g., mobile switching center (MSC)) or gateway (e.g., packet data serving node (PDSN)) that provides connectivity with a transport network, such as the public switched telephone network (PSTN) or the Internet. With this arrangement, a cell phone or other wireless communication device (generally “mobile station”) that is positioned within the coverage area of a given sector can communicate over an air interface with the BTS and in turn via the BSC and switch or gateway with entities on the transport network.
Unlike landline telephones that exist at known, fixed locations, mobile stations can operate at virtually any location where a wireless carrier provides radio-frequency coverage. Consequently, in order for a mobile station to be able to engage in useful communications (voice or data) in a cellular wireless communication system, the mobile station must first register with the system. This registration process lets the system know where the mobile station is located (e.g., for purposes of directing calls to the mobile station) and so that the system can verify that the mobile station is authorized to be operating in the system.
The manner in which a mobile station registers with a cellular wireless communication system can take various forms, depending on factors such as the configuration of the system and on the communication protocols used. For example, such registration may be accomplished using Code Division Multiple Access (“CDMA”). CDMA is described in further detail in Telecommunications Industry Association (“TIA”) standards IS-95A and IS-95B, which are both incorporated herein by reference in their entirety. CDMA is also described in the International Telecommunications Union (“ITU”) IMT-2000 series of standards, which are all incorporated herein by reference in their entirety. CDMA is further described in the TIA IS-2000 series of standards, which are all incorporated herein by reference in their entirety. The IS-2000 series of standards are commonly referred to as CDMA2000.
In a system operating according to the CDMA2000 protocol, for instance, a mobile station registers by sending over the air to the base station an “access probe,” which carries an identifier of the mobile station and perhaps other pertinent information. The mobile station sends the access probe in a “slotted aloha process.” In the slotted aloha process, the mobile station repeatedly sends the access probe at increasingly higher power levels until it receives an acknowledgement message from the base station, or until it otherwise exhausts the process (e.g., the maximum transmission power of the mobile station is reached and no acknowledgment has been received). As is described in the CMDA2000 specifications, each access probe travels in a timeslot of an air interface access channel from the mobile station to the base station, while each registration acknowledgement travels in a timeslot of an air interface paging channel from the base station to the mobile station.
When the base station receives an access probe from a mobile station, the base station passes the access probe along to the switch (e.g., an MSC) or other entity (e.g., a PDSN), which then responsively sends a registration notification message to the mobile station's home location register (HLR). The HLR then updates the mobile station's profile to indicate where the mobile station is operating (e.g., which switch is serving the mobile station) and may further carry out an authentication process, and then sends a registration response, which propagates to the mobile station. If the mobile station has registered with a base station that is not in its “home network” (e.g., operated by the wireless provider with which the mobile station is associated), the registration information for the mobile station is stored in a visitor location register (VLR) in the network in which the mobile station is operating (which may be termed “roaming”), as well as in an HLR in the mobile station's home network.
Various trigger events can cause mobile stations to register with the system. In a CDMA2000 system, for instance, a mobile station will generally register (i) whenever it enters a new zone (e.g., sector and/or cell) in response to a distinct “reg_zone” parameter the mobile station receives in an air interface control channel message from the base station, (ii) on a periodic basis, with a period indicated by a “reg_period” parameter (or directive) that the mobile station receives in an air interface control channel message from the base station, and (iii) when the mobile station receives or places a call.
In some situations, messages transmitted from the system to a mobile station may not successfully reach the mobile station. For example, a mobile station may have moved out of a particular zone (e.g. sector and/or cell), or the mobile station may be in a location where signals from the system are attenuated or subject to interference. A decrease in the successful transmission of messages from the system to a mobile station can negatively impact a customer's experience by causing calls to be directed to voicemail without notifying the user, delaying the receipt of SMS notifications, causing call set up latencies, and generally causing suboptimal performance of features that rely on messages being transmitted from the system to the mobile station.
In some situations, the air interface between a base station (e.g., a particular sector associated with the base station) and mobile stations operating within the coverage area of the base station can become overwhelmed with too much use. This can happen, for example, if too many mobile station registrations occur at once. In a CDMA2000 system, for instance, if access probes from two or more mobile stations line up (by chance) in the same timeslot of the access channel, an “access probe collision” occurs. The result of such a collision is that none of the probes will succeed, principally because the base station will not receive any of the probes in a comprehensible form due to interference between the multiple access probes. Thus, should such a collision occur, each mobile station would have to re-send its access probe, because it would not receive an acknowledgement from the base station.
In many situations, access probe collisions are not very likely to occur because sufficient timeslots exist on the access channel for transmitting the access probes. However, in situations where many users are placing calls at once, the number of access probes and access probe collisions can increase dramatically due to collisions and associated retries. For example, after a football game or in an emergency situation, many people within a given sector of a specific cell site may use their mobile phones to place calls (e.g., to call 911, to call friends and family, to check voice mail, or for other purposes).
Each time a mobile station places a call, as was noted above, the mobile station sends an access probe. Consequently, in a situation where many people within a given sector place calls at once, many access probes will be sent at once. In turn, access probe collisions then occur and, therefore, still more (retry) access probes are sent. Further, as these collisions and retries are occurring, other mobile stations in the sector are periodically registering with the system, according to the “reg_period” directive from the base station, which will still further increase the frequency of access probe collisions. In a CDMA2000 system, the reg_period directive includes the frequency with which mobile stations periodically register with the system. Such access probe collisions may result in mobile stations being unable to place calls, receive calls, send data and/or receive data, as the mobile stations may be unable to successfully register with the system due to the occurrence of access probe collisions.